Methods for rugged attachment of fibers to integrated optics chips and product thereof

ABSTRACT

Methods are disclosed for providing a rugged attachment of an optical fiber to an integrated optics chip. A first plate is attached to a first surface of the chip, the thickness of the chip is reduced to a value less than the diameter of the fiber but of sufficient optical thickness so that the evanescent field of a guided mode is negligible outside the chip, a second plate is attached to a second major surface of the reduced-thickness chip, the fiber is coaxially jacketed with a material suitable for attachment to the first and second plates, the fiber is positioned with respect to a waveguide on the chip, and the jacket of the fiber at its end face is symmetrically attached to both the first and second plates at a plurality of locations. Alternatively, a single plate can be attached to the first surface of the chip without reducing the thickness of the chip and either a jacket on the fiber or the curved periphery of the fiber itself can be attached symmetrically to both the plate and the substrate. The resulting connection between the optical fiber and the integrated optics chip is rugged, thermally insensitive, and suited for batch production processes. A wafer comprising a plurality of chips can be sandwiched between first and second plates or reinforced with a single plate, and subsequently subdivided into individual chips to which optical fibers can then be attached.

This application is a continuation-in-part of application Ser. No.07/579,302, filed Sept. 7, 1990, (abandoned) which is a division ofapplication Ser. No. 07/310,336, filed Feb. 13, 1989, now U.S. Pat. No.4,976,506, issued Dec. 11, 1990, by George A. Pavlath entitled "METHODSFOR RUGGED ATTACHMENT OF FIBERS TO INTEGRATED OPTICS CHIPS AND PRODUCTTHEREOF."

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to fiberoptic devices and techniquesand, more particularly, to methods for attaching fibers to integratedoptics chips and the devices resulting from such methods.

2. Description of the Related Art

Short lengths of optical fibers called "pigtails" are bonded tointegrated optics chips to aid in coupling to devices on the chips. Onceinstalled, a pigtail allows fiber-to-fiber splicing to be carried out.

Previous methods for attaching optical fibers to integrated optics chipsinvolve the use of epoxy adhesives or metal-to-metal welding. An exampleof the latter is the method disclosed in U.S. patent application Ser.No. 006,164, entitled "Microattachment of Optical Fibers," filed Jan.23, 1987, now U.S. Pat. No. 4,788,406, and assigned to the assignee ofthe present invention, which application is hereby incorporated byreference herein.

Epoxy bonds tend to be limited by the characteristics of the epoxy (forexample, by changes in light-transmitting and mechanical properties inthe desired operating temperature range), are slow to set up, and aredifficult to use in an automated production environment. Past methods ofmetal-to-metal welding have involved structurally asymmetric attachmentarrangements which result in undesirably high thermal sensitivities ofthe optical splices. In an asymmetric attachment, the disparity incoefficients of thermal expansion between adjacent dissimilar materialsresults in thermally-induced stresses on the fiber. It would be a greatadvance in the art of attaching optical fibers to integrated opticschips if a method could be found which is suited for batch processingand provides a joint which is symmetric and thermally insensitive.

SUMMARY OF THE INVENTION

A method for providing a rugged attachment of an optical fiber to anintegrated optics chip comprises attaching a first plate to a firstmajor surface of the chip substrate on which the optical circuitsreside, removing material from a second major surface of the chip toleave a chip thickness smaller than the diameter of the fiber to beattached to the chip but of sufficient optical thickness so that theevanescent field of a guided mode has a negligibly small value outsidethe chip, attaching a second plate to a second major surface of thereduced-thickness chip, coaxially jacketing an end portion of the fiberwith a material suited for attachment to the first and second plates,positioning an end face of the fiber with respect to the chip so thatthe fiber is properly optically coupled to a desired portion of an endface of the chip, and attaching the jacket of the fiber end facesymmetrically to both the first and second plates at a plurality oflocations.

Alternatively, a single plate can be attached to the first major surfaceof the chip without reducing the thickness of the chip, and the jacketof the fiber end face can be attached symmetrically to both the plateand the chip substrate at a plurality of locations. In this embodimentthe material of the plate must be the same as the material of the chipsubstrate. In a variation on this alternative embodiment, the singleplate and the chip substrate comprise materials which are suitable forattachment to the curved periphery of an unjacketed fiber, so that thefiber itself can be symmetrically attached to the plate and thesubstrate at a plurality of locations.

The resulting connection between the optical fiber and the integratedoptics chip is rugged, thermally insensitive, and suited for batchproduction processes. In particular, a wafer comprising a plurality ofchips can be sandwiched between first and second plates or reinforcedwith a single top plate, and subsequently subdivided into individualchips to which optical fibers are then attached by the method disclosed.

Techniques of attaching the sandwiching plates or reinforcing plateinclude using an adhesive, anodic bonding, and laser welding. Jacketingof the optical fiber can be accomplished either by sliding an end of thefiber into a coaxial sleeve of the jacketing material or by coating anend portion of the fiber with a coaxial layer of the material. Suitablecoating procedures include electroplating, electroless deposition, vapordeposition, and the application of the coating material in liquid form.

In the embodiment in which the integrated optics chip is reduced inthickness, removal of material from the chip can be done by polishing.Monitoring of the thickness of the chip during the polishing process canbe effected in a variety of ways including electro-acoustic techniques.

BRIEF DESCRIPTION OF THE DRAWINGS

The various features and advantages of the present invention will becomeapparent from the following detailed description taken together with theaccompanying drawings, in which:

FIG. 1 is a top plan view of an integrated optics chip which is to bepigtailed; the chip contains a Y circuit typically used in fiberopticgyroscopes;

FIG. 2 is an end elevational view of the chip of FIG. 1;

FIG. 3 is an end elevational view of an integrated optics chip with afirst plate attached in accordance with the invention using an epoxyadhesive;

FIG. 4 is an end elevational view of an integrated optics chip with afirst plate attached according to the invention using an anodic bondingtechnique;

FIG. 5 is an end elevational view of a chip with plate attached afterpolishing of the chip to remove material;

FIG. 6 is an end elevational view of an integrated optics chip with asecond plate attached to the chip after its thickness has been reduced;

FIG. 7 is an end elevational view of an optical fiber jacketed accordingto the present invention;

FIG. 8 is a side elevational view, in section, of an optical end beinginserted into a sleeve of jacketing material;

FIG. 9 is a side elevational view of a jacketed optical fiber attachedto an end face of a sandwiched integrated optics chip according to thepresent invention;

FIG. 10 is a sectional view as indicated in FIG. 9;

FIG. 11 is a side elevational view of a jacketed optical fiber attachedto a reinforced integrated optics chip according to the presentinvention;

FIG. 12 is a sectional view as indicated in FIG. 11;

FIG. 13 is an end elevational view of a sandwiched integrated opticswafer; and

FIG. 14 is an end elevational view of a reinforced integrated opticswafer.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a top plan view of an integrated optics (IO) chip 2, whichis to be pigtailed, comprising an optical Y circuit 4 on a substrate.The Y optical circuit 4 is of a type typically used in fiberopticgyroscopes. The standard circuit has an acoustic transducer 6 placed inan unused portion of the chip 2. Transducer 6 consists of an electrodepattern which is designed to launch acoustic waves into the substrate ofchip 2 over a broad bandwidth (a few hundred MHz to 1 GHz). Such anelectrode pattern is deposited via conventional photolithographic andvacuum deposition techniques on the substrate of the chip. The spacing 8between the branches of Y circuit 4 at an end face 11 of chip 2 istypically on the order of 200 to 400 micrometers.

FIG. 2 shows an end elevational view of IO chip 2 as seen from the rightin FIG. 1. Optical waveguide end faces 12 and 14 are the terminations ofthe two branches of Y circuit 4 at the end face 11 of chip 2. In thesefigures the center-to-center distance 8 can be about 350 micrometers,the height 16 of chip 2 can be about 1 mm, and the width 18 of chip 2can be about 3 mm.

The first step in a method for providing rugged attachment of an opticalfiber to integrated optics chip 2 is to attach a suitable glass,ceramic, or metal plate 20 to the top surface of chip 2 as shown in FIG.3, substantially flush with end face 11. Various bonding techniqueswhich include the use of epoxy glues and anodic techniques can beemployed. The use of an epoxy adhesive to bond plate 20 to the topsurface of chip 2 is illustrated in FIG. 3, where reference numeral 22represents a thin layer of adhesive between plate 20 and chip 2.

FIG. 4 illustrates the use of an anodic bonding technique to join plate20 with chip 2. In anodic bonding, a suitable dielectric buffer layer 24is deposited on the top surface of chip 2. Dielectric film 24 istypically about 5,000 angstroms thick. A metal plate can then be bondedto dielectric film 24 by applying pressure between the plate and thefilm and simultaneously applying an electric field to produce migrationof metal atoms into the film.

The next step in the process of the invention is to reduce the thicknessof chip 2 by removing material from bottom surface 26. One method fordoing this would be to polish bottom surface 26. The resulting thicknessshould be less than a fiber diameter but should be of sufficient opticalthickness to contain the guided mode with negligible "leakage" from thewaveguide. In other words, the evanescent field of the guided modeshould have negligible strength outside the bottom surface 26 of chip 2,where "negligible" means that the light loss can be tolerated in aparticular application.

FIG. 5 shows an end elevational view of a plate-chip combination withreduced-thickness chip 28 having a final thickness of about 10micrometers. The thickness of reduced-thickness chip 28 can be measuredby determining the acoustic resonance frequencies of the transducerdeposited in the beginning of the process or by several othertechniques.

Next, as shown in FIG. 6, a second metal, glass, or ceramic plate 29similar in dimensions to first plate 20 in dimensions is attached to thebottom surface of reduced-thickness IO chip 28 using the same techniqueas was used to attach first plate 20. The resulting sandwiched chip 30is then ready for attachment of an optical fiber.

Before attachment to sandwiched chip 30, an optical fiber 32 is preparedas follows. As shown in FIG. 7, which is an end view of a fiber preparedfor chip attachment, the outer surface of fiber 32 must be coated with amaterial that is capable of being laser welded, E-beam welded, orotherwise attached to plates 20 and 29 which form a sandwich ofreduced-thickness chip 28. The metal, glass, or ceramic jacket 34 can beformed on fiber 32 in a variety of ways. Jacket 34 may be flush with theend of fiber 32 or it may protrude beyond the end in a clip-likestructure. The fiber shown in Fib. 7 is a particular type ofpolarization maintaining (PM) fiber (Corning PRSM (for "polarizationretaining single-mode")) but any type of fiber can be used in generalwith this technique. Fiber core 36 and stress members 38 and 40 arecontained in cladding 42 of fiber 32.

In the case of a metal jacket, a jacket coating 34 can be applied by avapor transport process (such as by evaporation), by electroplating, byan electroless process, or by fitting a coaxial sleeve of the metal overthe fiber. A coaxial sleeve arrangement also works well with othermaterials such as glasses, plastics, or ceramics. FIG. 8 is a side view,in section, of an optical fiber end being inserted into a sleeve ofjacketing material.

As shown in the side view of FIG. 9, a jacketed fiber end face is nextpositioned adjacent a narrow end face of sandwiched chip 30 so thatlight from a waveguide 12 can properly transmit light to or receivelight from fiber core 36. The end of jacket 34 on fiber 32 is thenjoined (for example, by welding) to the narrow end faces 44 and 46 oftop plate 20 and bottom plate 29, respectively. The welding is donesymmetrically with respect to a plane through the center of fiber 32parallel to either surface of chip 28. One possible symmetric pattern ofwelds is shown in the sectional view of FIG. 10. The plurality ofsymmetrical welds 48 comprise a rugged attachment of fiber 32 to chip 28which will not result in thermal stresses on either fiber 32 or chip 28when the ambient temperature changes.

The technique described above is well suited to attaching fibers tosingle-crystal integrated optics chips such as those made of lithiumniobate (LiNbO₃), gallium arsenide (GaAs), and potassium titanylphosphate (KTP). An alternative technique in accordance with theinvention can be used for attaching a fiber (jacketed or unjacketed) toa glass substrate. In this alternative process the attachment of only atop plate is required, as shown in FIG. 3. The step of polishing thebottom surface of the substrate to remove material is unnecessary, as isattachment of a bottom plate.

FIG. 11 shows a side elevational view of chip substrate 2 reinforcedwith a top plate 20, and an optical fiber 32 (jacketed or unjacketed)positioned next to substrate 2 and plate 20 in preparation forattachment. In the sectional view of FIG. 12, optical fiber jackets 34aand 34b are shown attached to top plate 20 and substrate 2 by aplurality of welds 48 at symmetrical locations with respect to a planeparallel to the top surface of substrate 2 which passes through thecenters of fiber cores 36a and 36b. The fibers 32a and 32b in thisalternative technique need not be jacketed if their curved surfaces aremade of a material which can be directly attached to top plate 20 andchip substrate 2.

The methods described above produce stable, thermally rugged, andsymmetric attachments between IO chips and coated or uncoated opticalfibers.

Fibers and waveguides can be of any type including rib waveguides. For arib waveguide or an electroded chip, a layer is placed on the topsurface of the chip, and perhaps polished, to assure a flat surface forattachment of a top plate.

The processes of the invention can be carried out on chips and onwafers. A wafer comprising a plurality of integrated optics chips can beattached to a top plate, reduced in thickness, and then attached to abottom plate as described above. The sandwiched, reduced-thickness wafercan subsequently be cut into individual IO chips for attachment ofoptical fibers in the manner described above.

FIG. 13 is a side elevational view of a sandwiched wafer 50 beforesubdivision into individual chips. Similarly, a wafer comprising aplurality of IO chips on a glass substrate can be attached to a singletop plate without being reduced in thickness, as shown in FIG. 14. Theresulting reinforced wafer 52 can then be cut into a plurality ofindividual IO chips to which optical fibers can be attached by themethods described above. These wafer methods are well suited forfabrication on a production basis.

The rugged optical fiber attachments and thermally insensitiveconnections produced in accordance with the invention are well matchedto single-crystal and amorphous IO substrates. The choice of materialfor the top and bottom plates covers a wide range (glasses, ceramics,and metals), with the bonding technology being chosen for the particularselections of plate and substrate materials. Since the bonding methodscan be applied to chips or wafers, fabrication of chip-fiber devices canbe done readily at high rates of production. The techniques describedabove permit rapid attachment of multiple fibers to a single end face ofan integrated optics chip. The multiple fibers can be located in closeproximity to each other, a feature which is vitally important for fiberoptic gyroscope manufacture.

The above-described embodiments of the invention are furnished asillustrative of its principles, and are not intended to define the onlyembodiments possible in accordance with our teaching. Rather, theinvention is to be considered as encompassing not only the specificembodiments shown, but also any others falling within the scope of thefollowing claims.

What is claimed is:
 1. A method for attachment of a jacketed opticalfiber to an integrated optics chip comprising:(a) attaching a firstsurface of a first plate to a first surface of a substrate of said chiphaving a light conduit thereon ending at an end face thereof so that anend face of said first plate extends substantially to said end face ofsaid chip, said chip having a thickness less than the diameter of saidjacketed fiber; (b) attaching a first surface of a second plate to asecond surface of said chip, an end face of said second plate extendingsubstantially to said end face of said chip; (c) positioning an end faceof said jacketed fiber adjacent to said end face of said chip so thatsaid fiber can properly transmit light to and receive light from saidconduit; and (d) attaching the jacket of said jacketed optical fiber atsaid end face of said fiber to said end faces of said first and secondplates at a plurality of locations.
 2. The method of claim 1 whereinsaid locations are symmetrically disposed with respect to the centralplane of said chip, said plane being parallel to said first and secondsurfaces of said chip.
 3. The method of claim 1 wherein said attachingin step (a) is done with an adhesive.
 4. The method of claim 1 whereinsaid attaching in step (d) is done using an adhesive.
 5. The method ofclaim 1 wherein said attaching in step (a) is done by anodic bonding. 6.The method of claim 1 wherein said attaching in step (b) is done byanodic bonding.
 7. The method of claim 1 further comprising removingmaterial from said second surface of said chip as necessary to form saidthickness less than the diameter of said fiber.
 8. The method of claim 1comprising jacketing said fiber by sliding an end of said fiber into acoaxial sleeve open at both ends thereof.
 9. The method of claim 1further comprising jacketing said fiber by coating said fiber with acoaxial layer.
 10. The method of claim 9 wherein said coating is done byan electroplating process.
 11. The method of claim 9 wherein saidcoating is done by an electroless deposition process.
 12. The method ofclaim 9 wherein said coating is done by a vapor deposition process. 13.The method of claim 9 wherein said coating is done by applying a liquidto said fiber.
 14. The method of claim 1 wherein said jacketing extendsslightly beyond said end face of said fiber.
 15. The method of claim 1wherein said jacket is metallic and said attaching in step (d) is doneby laser welding.
 16. A method for attachment of a jacketed opticalfiber to an integrated optics chip comprising:(a) attaching a firstsurface of a first plate to a first surface of a substrate of said chiphaving a light conduit thereon ending at an end face thereof so that anend face of said first plate extends substantially to said end face ofsaid chip, said chip having a thickness less than the diameter of saidjacketed fiber; (b) attaching a first surface of a second plate to asecond surface of said substrate, an end face of said second plateextending substantially to said end face of said chip; (c) positioningan end face of said jacketed fiber adjacent to said end face of saidchip so that said fiber can properly transmit light to and receive lightfrom said conduit; and (d) attaching the jacket of said jacketed opticalfiber at said end face of said fiber to said end faces of said first andsecond plates at a plurality of locations.
 17. The method of claim 1wherein said chip comprises a monocrystalline material chosen from thegroup consisting of lithium niobate (LiNbO₃), gallium arsenide (GaAs),and potassium titanyl phosphate (KTP).
 18. The method of claim 1 whereinsaid chip comprises a glass.
 19. The method of claim 1 wherein saidthickness of said chip corresponds to an optical thickness that issufficient to make the evanescent field to a guided mode negligiblysmall outside said chip.
 20. A method for attachment of at least onejacketed optical fiber to an integrated optics chip comprising:(a)attaching a first surface of a plate to a first surface of a substrateof said chip having a light conduit thereon ending at an end facethereof so that an end face of said plate extends substantially to saidend face of said chip, said plate comprising a material the same as thatof said substrate, said chip having a thickness less than the diameterof said fiber; (b) positioning an end face of said jacketed fiberadjacent to said end face of said chip so that said jacketed fiber canproperly transmit light to and receive light from said conduit; and (c)attaching said jacket at said end face of said fiber to said end facesof said substrate and said plate at a plurality of locations.
 21. Themethod of claim 20 further comprising disposing said locations in step(c) symmetrically with respect to the plane in which said first surfaceof said chip substantially lies.
 22. The method of claim 16 furthercomprising removing material from said second surface of said substrateas necessary to form said thickness less than the diameter of saidfiber.
 23. The method of claim 20 wherein said attaching in step (a) isdone by an anodic bonding.
 24. The method of claim 20 wherein aidsubstrate comprises a glass and said material also comprises a glass.25. The method of claim 20 wherein said attaching in step (c) is done bylaser welding.
 26. The method of claim 20 wherein said jacketing isformed by sliding an end of said fiber into a coaxial sleeve of saidmaterial open at both ends thereof so that said end face of said fiberextends substantially to an end of said coaxial sleeve.
 27. The methodof claim 20 wherein said jacketing is formed by sliding an end of saidfiber into a coaxial sleeve of said material open at both ends thereofso that said end face of said fiber is nearly flush with an end of saidsleeve, said sleeve extending beyond said end of said fiber.
 28. Themethod of claim 20 wherein said jacketing is formed by coating saidfiber with said material.
 29. The method of claim 28 wherein saidcoating comprises applying said material as a liquid.
 30. The method ofclaim 28 wherein said coating comprises depositing said material by avapor transport process.
 31. A method for facilitating the attachment ofjacketed optical fibers to integrated optics chips comprising:(a)starting with a wafer comprising a plurality of integrated optics chipson a first surface of a substrate, attaching a first surface of a firstplate to said first surface of said wafer so that a peripheral edge ofsaid first plate is substantially flush with a peripheral edge of saidwafer substrate, said substrate having a thickness of less than thediameters of said optical fibers; (b) attaching a first surface of asecond plate to a second surface of said wafer so that said peripheraledge of said substrate is substantially flush with a peripheral edge ofsaid second plate; (c) subdividing a sandwiched wafer resulting fromstep (b) into a plurality of individual sandwiched integrated opticschips; (d) positioning an end face of each said fiber adjacent to an endface of each said sandwiched chip so that said fiber can properlytransmit light to and receive light from a light conduit of saidsandwiched chip ending at said end face; and (e) attaching each of aplurality of jackets at each of said end faces of said plurality offibers positioned at each of said end faces of said plurality ofsandwiched chips to said first and second plates at a plurality oflocations.
 32. The method of claim 16 wherein said substrate comprises amonocrystalline material chosen from the group consisting of lithiumniobate (LiNbO₃), gallium arsenide (GaAs), and potassium titanylphosphate (KTP).
 33. The method of claim 16 wherein said substratecomprises a glass.
 34. The method of claim 16 wherein said thickness ofsaid substrate corresponds to an optical thickness that is sufficient tomake the evanescent field to a guided mode negligibly small outside saidchip.
 35. The method of claim 31 wherein said attaching in step (a)comprises anodic bonding.
 36. The method of claim 31 wherein saidattaching in step (b) comprises anodic bonding.
 37. The method of claim31 further comprising removing material from said second surface of saidwafer substrate as necessary to form said thickness less than saiddiameters of said optical fibers.
 38. The method of claim 31 whereinsaid thickness of said chip corresponds to an optical thickness that issufficient to make the evanescent field of a guided mode negligiblysmall outside said chip.
 39. A method for facilitating the attachment ofjacketed optical fibers to integrated optics chips comprising:(a) to awafer comprising a plurality of integrated optics chips on a firstsurface of a substrate, attaching a first surface of a plate comprisinga material the same as that of said substrate of said wafer to saidfirst surface of said wafer so that a peripheral edge of said plate issubstantially flush with a peripheral edge of said wafer substrate; (b)subdividing a reinforced wafer resulting from step (a) into a pluralityof individual reinforced integrated optics chips; (c) positioning an endface of each said fiber adjacent to an end face of each said reinforcedchip so that said fiber can properly transmit light to and receive lightfrom a light conduit of said reinforced chip ending at said end face;and (d) attaching each of a plurality of jackets at each of said endfaces of said plurality of fibers positioned at each of said end facesof said plurality of reinforced chips to said substrate and to saidplate at a plurality of locations.
 40. The method of claim 39 whereinsaid locations in step (d) are symmetrically disposed with respect tothe central plane of each of said reinforced chips, said plane beingparallel to said first and second surfaces of said chip.
 41. The methodof claim 39 wherein said attaching in step (a) comprises using anadhesive.
 42. The method of claim 39 wherein said attaching in step (a)comprises anodic bonding.
 43. The method of claim 39 wherein saidsubstrate comprises a glass and said material comprises a glass also.44. The method of claim 39 wherein said attaching in step (d) compriseslaser welding.
 45. The method for attachment of at least one opticalfiber having a curved surface to an integrated optics chipcomprising:(a) attaching a first surface of a plate to a first surfaceof a substrate of said chip having a light conduit thereon ending at anend face thereof so that an end face of said plate extends substantiallyto said end face of said chip, said plate comprising a material the sameas that of said substrate, said chip having a thickness less than thediameter of said at least one optical fiber; (b) positioning an end faceof said fiber adjacent to said end face of said chip so that said fibercan properly transmit light to and receive light from said conduit; and(c) attaching said fiber at said end face of said fiber to said endfaces of said substrate and said plate at a plurality of locations, saidcurved surface of said at least one fiber being made of a material whichcan be directly attached to said end faces of said substrate and saidplate.